Hitherto, the particulate fluidising media have been supported on a diffusion tile, which tile may be made from a porous ceramic or porous metal, or from a metal plate punched or machined with a plurality of apertures. Difficulties can, however, arise in using such diffusion tiles. For example, all such tiles are difficult to seal around their periphery and it is not easy to replace damaged or worn tiles. Furthermore, problems arise from differential thermal expansions which may give rise to cracked ceramic tiles or buckled metal tiles. Also holes or apertures in metal tiles are frequently clogged by the fluidising media or by solid residues in some fluidising gases. Yet another disadvantage of using diffusion tiles is that it is not possible to keep separate different components of the fluidising gas before it enters the bed. Such separation is necessary if the gaseous components form an ignitable or explosive mixture because light-back may occur through the diffusion tile or the tile may be overheated as a result of flame burning on its surface.
To overcome some of the aforementioned disadvantages it has previously been proposed to make an apertured tube or sparge pipe assembly located within the container adjacent to its base and connected to a source of fluidising gas disposed outside the container. Such an arrangement possesses disadvantages. When in a non-fluidising or rest condition some of the particulate fluidising material may enter the sparge pipe via the apertures and thereby subsequently impair the efficiency of the distribution of fluidising gas and in some cases actually block all or part of the sparge pipe. Another disadvantage results from the fact that, in order to avoid lateral pressure drops in the length of the sparge pipe, the fluidising gases are introduced to said pipes at a plurality of points along their length by supply tubes which are fixed to the relatively cool base of the container, and this arrangement results in differential thermal movements of the sparge pipe relative to the supply pipes which, in turn, gives rise to distortion and cracking of the assembly. Yet another problem is that the sparge pipe is usually unevenly heated so that, for example, the top of the pipe in the fluidising area of the bed becomes hotter than the bottom of the pipe adjacent to the cooler container base. This causes distortion of the sparge pipe along its lateral axis. Yet another disadvantage is that particulate fluidising material in a deep bed suffers from a hysteresis effect at the onset of fluidisation. This is caused by initial differential pressures along the length of the sparge pipe, so that gases emitted from apertures adjacent to high pressure zones in the pipe will clear a channel in the particulate fluidising material, thus reducing the pressure drop of gases exiting from said apertures. This initiates a fluidic or directional gas flow effect so that gases preferentially exit in apertures of low pressure drop while the remaining apertures in the sparge pipe do not clear and thereby do not achieve an adequate flow, thereby causing uneven fluidisation of the bed.
One object of the present invention is therefore to provide an improved fluidised bed which avoids the above-mentioned disadvantages.